Motor vehicle transmissions generally include selectively engageable gear elements for providing two or more forward speed ratios through which engine output torque is applied to the vehicle drive wheels. In automatic transmissions, the gear elements which provide the various speed ratios are selectively activated through fluid operated torque establishing devices such as clutches and brakes. The brake can be of the band or disk type; engineering personnel in the automotive art refer to the disk type brakes in transmissions as clutches or clutching devices. Thus, shifting from one speed ratio to another generally involves releasing (disengaging) the clutch associated with the current speed ratio and applying (engaging) the clutch associated with the desired speed ratio. The clutch to be released is referred to as the off-going clutch, while the clutch to be applied is referred to as the on-coming clutch. There is generally a slight overlap between the release and apply, and high quality shifts are only achieved when the release and apply are properly timed and executed.
Conventionally, the control of shifting in an automatic transmission is performed with hydraulic logic and servo elements responsive to various system parameters such as vehicle speed and throttle position. Fluid pressure signals representative of the various system parameters are processed to determine when a shift is in order, and spring elements and fluid orifices within the servo elements determine the timing calibration for the release and apply of the respective clutching devices.
To overcome certain disadvantages of hydraulic control, it has been proposed to electronically perform at least some of the transmission control functions. For example, it has been suggested to electronically determine the desired speed ratio based on measured system parameters, and directly control the supply of fluid to the respective clutching elements to effect shifting from one speed ratio to another. Among the advantages of electronic control are reduced hardware complexity, increased reliability and greater control flexibility. An example of an electronic control system for an automatic transmission is given in the Marlow U.S. Pat. No., 3,688,607 issued Sept. 5, 1972, which patent is assigned to the assignee of the present invention.
The U.S. Pat. No. 3,688,607 referred to above discloses a closed loop control where the speed rate of change of a specified transmission element is made to conform with a reference rate. The present invention, on the other hand, is directed to an open loop control. In open loop control, the fluid valves are controlled in accordance with a predetermined schedule to effect apply and release of the various transmission clutching devices, and the control is not changed in the course of a shift in accordance with a measure of a controlled parameter. Such pure open loop control of the clutching devices is acceptable so long as there are no significant variations in the engine and transmission operating characteristics, and no significant assembly tolerances. However, engine and transmission operating characteristics do change with time, and the production assembly tolerances may result in significant vehicle-to-vehicle variability. As a result, control schedules that produce acceptable ratio shifting in one vehicle may produce unacceptable ratio shifting in another vehicle.